This invention is directed to the field of coating sheet metal and more particularly, to a process for coating sheet metal that does not result in the emission of pollutants that need to be removed before the emission is released into the atmosphere.
The traditional method of coating sheet metal is to apply a solvent or water-based coating to the sheet metal, and then move the sheet metal through an oven to cure the coating. The curing is accomplished by heating the sheet metal and the coating to a temperature at which the solvent or water in the coating is evaporated and at which the coating itself is not harmed. To obtain a uniform coating and for greatest efficiency and lowest cost, the curing is a continuous process in which 1) the oven needs to be maintained at a constant temperature, 2) the sheet metal needs to be continuous and 3) the sheet metal needs to be moved through the oven at a constant rate of speed. A typical rate is 200 to 400 feet per minute.
To meet the requirement that the sheet metal be continuous, two pay off coils of sheet metal and a welder/joiner are typically provided. The sheet metal is advanced from one of the pay off coils, and when the end of that coil is reached, it is welded to the beginning of the second pay off coil. Then, as sheet metal is advanced from the second coil, the exhausted first coil is replaced. In addition, after the coating on the sheet metal is cured, the sheet metal needs to be re-coiled. To accomplish this, a shear and two re-coil mechanisms are typically provided. The continuously moving sheet metal is wound on a first re-coil mechanism, and when the desired coil size is reached, the sheet metal is cut by the shear. The sheet metal is then wound on the second re-coil mechanism while the coil on the first re-coil mechanism is removed.
Both the welding/joining operation at the beginning of the process and the shearing operation at the end of the process require interruption of the movement of the sheet metal. Therefore, to satisfy the requirement that the sheet metal move through the oven at a constant rate of speed, it is necessary that a first excess length of sheet metal be provided after the welder/joiner. This excess length of sheet metal is fed into the process during the time that the welding/joining operation is taking place and no sheet metal is being advanced from one of the pay off coils. It is apparent that even at the modest rate of speed of 200 to 400 feet per minute, allowing for the worst case interruption requires that the excess length of sheet metal be substantial.
This excess length of sheet metal is provided by apparatus referred to as an accumulator. This accumulator is typically a tower within which the excess sheet metal is vertically looped back and forth on itself in serpentine fashion. The ends of the loops are wrapped around rollers that move toward one another to shorten the loops when excess length is being used to replace the sheet metal not being provided by a pay off coil. Once the welding/joining operation is completed, the rollers move away from one another as the desired excess length is restored in the accumulator.
A second accumulator needs to be provided before the shear and re-coil mechanisms. This is because the sheet metal is continuously moving out of the oven at the constant rate of speed. During the time when the winding of the sheet metal is being transferred from one of the re-coil mechanisms to the other, the sheet metal needs to be accumulated. In the second accumulator, the length of the loops are increased when no re-coil mechanism is in operation and decreased when a re-coil mechanism is in operation.
The oven used in the traditional method needs to be of considerable length in order to effect complete curing of the coating applied to the sheet metal. A length of 100 feet for sheet metal of 0.050 thickness moving at a rate of 200 to 400 feet per minute is appropriate. Of course, the length of the oven needs to be increased if the thickness of the sheet metal and/or the rate at which the sheet metal moves through the oven is increased.
Prior to the coating being applied to the sheet metal, the sheet metal needs to be cleaned to remove contaminants that may interfere with the coating adhering to the sheet metal and pretreated to promote adhesion of the coating to the sheet metal. In the traditional method the materials used in the cleaning of the sheet metal contain pollutants in the form of volatile organic compounds that are emitted during the cleaning operation. In addition, the materials used in the pretreating of the sheet metal may also contain such pollutants. These pollutants need to be removed before the emissions can be released to the atmosphere.
Similarly, the coating used in the traditional method contain volatile organic compounds and hazardous air pollutants that are emitted during the curing operation. Again, these pollutants need to be removed before the emissions can be released to the atmosphere. It is, therefore, necessary in the traditional method to have in place pollution control equipment that removes these pollutants from the emissions exhausted from the cleaning operation, the pretreating operation, and the curing operation.
It is seen from the above that the traditional method for coating sheet metal has many deficiencies. First, it requires large amounts of equipment and a building of substantial size to house the equipment. Thus, it requires a significant investment of capital. Second, without a substantial increase in capital investment, it is a relatively low speed process. Third, it only lends itself to long runs. A coating line needs to operate around the clock for days or weeks once coating of sheet metal with a coating of a particular color has begun. Fourth, because of this and the many pieces of equipment involved in this process, the cost of operation and maintenance is significant. Last, and most importantly, it requires the installation of pollution control equipment to prevent pollution of the atmosphere.
The sheet metal coating process, in accordance with the present invention, provides very significant advantages over the traditional coating method.
The process, in accordance with the present invention, uses an electron beam curable coating rather than a heat curable coating. Consequently, the curing oven is eliminated along with the need to have a continuous length of sheet metal moving at a constant rate of speed. As a result, the second pay off mechanism, the two accumulator towers, and the second re-coil mechanism are all unnecessary. If individual sheets rather than a coil is being coated, then the welder/joiner and shear can also be eliminated. The equipment necessary to carry out the sheet metal coating process in accordance with the present invention is, therefore, far less costly than that required for the traditional method. In addition, because there is less equipment than in the traditional method, the cost of operating and maintaining the equipment is also reduced.
Another advantage of the coating process in accordance with the present invention is the rate of speed at which the sheet metal can be coated. Without increasing the cost of the equipment from that stated above, the sheet metal can be coated and cured at a rate of speed of 600 to 800 feet per minute. To provide this same rate with the traditional method would result in the equipment cost almost doubling.
Still another advantage of the coating process in accordance with the present invention is that it can accommodate not only long production runs, but also short ones. With the coating used in the traditional method, the coating cures even at ambient temperatures. The coating used in the present invention, on the other hand, only cures when it is exposed to an electron beam. Thus the system can be purged, the coating recovered, and the equipment that applies the coating to the sheet metal readily cleaned. Consequently a single coil can provide a continuous length of sheet metal and the movement of the sheet metal can be interrupted. The process in accordance with the present invention can, therefore, be used to do short custom runs. It can also be used to provide coated sheet metal in small batches on a just-in-time basis. This allows the purchaser to reduce their inventory and save money.
Most importantly, the coating process of the present invention is environmentally friendly in that it does not result in the emission of pollutants. The term xe2x80x9cpollutantxe2x80x9d as used in this patent is defined to mean anything characterized by the U.S. Environmental Protection Agency (EPA) as an air pollutant that exceeds limits established by the EPA.
The cleaning of the sheet metal to remove contaminants, in accordance with one embodiment of this invention, is accomplished ultrasonically. This is a water based, rather than solvent based, technology, and biodegradable aqueous detergents are available as an additive to the water washing solution. Following the washing of the sheet metal, it is given a clean water rinse and then dried. This cleaning operation does not result in the emission of pollutants. Another example of a non-solvent based cleaning technology is high turbulence circular flow nozzles which apply a water based cleaning solution to the sheet metal.
Pretreatment of the sheet metal is typically performed to promote adhesion of the coating to the sheet metal, and in accordance with one embodiment of this invention, the pretreatment is accomplished using a water based, rather than a solvent based, pretreatment solution that does not emit pollutants.
Pretreatment solutions when dried may also improve corrosion resistance . But where corrosion resistance is important to the product in which the coated sheet metal is to be used, a primer may be applied to the sheet metal subsequent to the pretreatment of the sheet metal. While the primary function of the primer is to enhance corrosion resistance, in some cases it also enhances adhesion of the coating to the sheet metal. Consequently, in some applications the primer may be used in place of the pretreatment. The primer in accordance with the present invention is either water based or electron beam curable.
Finally, it has been found that some electron beam curable top coatings have sufficient adherence directly to the cleaned sheet metal that for some applications no pretreatment or primer need be applied.
Coating of the sheet metal is accomplished using electron beam technology rather than an oven which cures the coating by evaporating the liquid in the coating. During the electron beam processing, molecules in the coating are irradiated with electrons and the coating is transformed into a solid through the process of polymerization and crosslinking. The transformation of the coating into a solid is virtually instantaneous and produces no emissions.
Thus, it is seen that the cleaning, pretreating and/or priming of the sheet metal and the curing of the coating does not result in the emission of pollutants that need to be removed before the emission is released to the atmosphere.